An aqueous coating composition comprising water insoluble resin microparticles uniformly dispersed in a system comprising water and an aqueous resin is being watched with keen interest in respect to prevention of air-pollution and fire, economization in expensive, hazardous organic solvent, and capability of forming various combinations of resins so as to attain the characteristic properties of the respective member resins. However, such a system comprising, as film-forming components, an aqueous resin and water insoluble resin microparticles, though being excellent in application characteristics in respects of sagging and pinholes under low to intermediate temperature conditions, has a problem in smoothing in the film-forming step and moreover, it can not give any solution on the problem of coating failures under high temperature coating conditions.
To cope with the abovementioned problems, the present inventors have provided a coating composition comprising water insoluble resin microparticles uniformly dispersed in an aqueous resin, said water insoluble resin being prepared by the reaction of a carboxyl containing resin which is solid at room temperatures and a mono-epoxy compound or monoisocyanate compound, and a Japanese application was filed on it (now laid open as Kokai No. 141249-58).
According to that invention, since a highly viscous coating is obtained and the coating viscosity is advantageously lowered at the initial stage of baking, the thus formed coating is excellent in that sagging and pinhole defects are effectively obviated. However, since the flowability is not so high, there is room for further improvements in face roughness and gloss of the finished coating.
With the intention of further improvements in gloss and the like, the present inventors have also provided a coating composition comprising water insoluble resin microparticles containing a resin which is liquid at room temperatures, uniformly dispersed in an aqueous resin system, and a Japanese patent application was filed on that invention (now laid open as Kokai No. 23330-61). Though the invention is likewise useful, the disclosed composition still has the problem such that sagging is liable to occur due to an excessive lowering in viscosity of the coating at the initial stage of baking thereof.
Under the circumstances, a coating composition has been desired which will satisfy the conflicting sagging and pinhole requirements in application characteristics, as well as roughness and gloss requirements in finished coating appearance.
Should a coating composition be obtained in the form of aqueous paint, containing as essential components, water, aqueous resin and water insoluble resin, those resins being selectable from various species and the composition being able to give a coating with excellent face roughness and gloss and free from sagging and pinhole defects, it might be useful as a top coat, an intermediate coat or an under coat for automobile bodies, electric appliances, household furniture and other general industrial uses, and especially as a top coat for automobile bodies. It is, therefore, an object of this invention to provide such coating composition.